In a reflecting electrode for a solar battery (device) in the past, a silver (Ag) film or an Ag alloy film having electrically low resistance and high optical reflectance (hereinafter referred to as reflectance) is used. For example, Patent Literature 1 discloses a solar battery in which a monocrystal or polycrystal silicon substrate including a back electrode of silver or copper is used. Paste containing aluminum as a main component is applied to the monocrystal or polycrystal silicon substrate and baked. Thereafter, an aluminum silicon alloy layer is removed. An electrode of silver or copper is formed on an exposed surface of the substrate by a vacuum evaporation method, a sputtering method, or the like. In this solar battery, an effect of reflecting incident light from a front surface on a rear surface using a back electrode layer of silver or copper having a high reflectance is remarkable, leading to an increase in an output current and an improvement of a photoelectric conversion efficiency.
However, the Ag film and the Ag alloy film have a problem in that adhesion to the substrate is poor and the film peels during a patterning process. Further, the Ag film and the Ag alloy film have a problem in that corrosion resistance against heating and the like is low and, for example, because of oxidation of the film surface due to exposure in the atmosphere or heat treatment, a reflectance falls and a specific resistance value increases.
To solve such problems, for example, Patent Literature 2 discloses a structure in which a layer containing Ag as a main component and containing lead (pd) is provided as a back electrode of a thin-film solar battery. The content of Pd on the front surface side is reduced to suppress deterioration in a reflection effect.
Patent Literature 3 discloses a back electrode of a solar battery in which a transparent conductive film, Ag, and an aluminum (Al) or Al alloy are laminated in this order to suppress a fall in reflectance due to, for example, whitening of Ag. The back electrode is an electrode in which the thickness of Ag is equal to or larger than 60 nanometers and the thickness of Al or Al alloy is 1 to 20 nanometers.
Although not a solar battery, for example, a technology for forming an Ag alloy reflection film and an adhesive layer between an Ag alloy and a substrate is disclosed in, for example, Patent Literature 4 and Patent Literature 5.
Patent Literature 4 discloses, as an Ag alloy reflection film having all of high reflectance, heat resistance in a process, corrosion resistance, and adhesion to a substrate, an Ag alloy reflection film containing 0.2 at % to 5 at % in total of one or more kinds of elements selected out of cerium (Ce), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), and dysprosium (Dy) and the remainder substantially formed of Ag.
Patent Literature 5 discloses that nitrogen content of a base substrate in an interface section between an Ag alloy film and the base substrate is increased to improve adhesion to the base substrate. As a method of realizing such a base substrate, Patent Literature 5 discloses that the surface of the base substrate is subjected to nitriding treatment in advance through reverse sputtering by nitrogen plasma or nitrogen gas. Further, Patent Literature 5 discloses that a large amount of nitrogen is contained in an interface portion of the Ag alloy film with a gate insulating film to prevent oxidation of the Ag alloy film. As a method of realizing such an Ag alloy film, Patent Literature 5 discloses that the film surface of the Ag alloy film is subjected to reverse sputtering by nitrogen plasma or nitrogen gas after formation of the Ag alloy film.